Why Size Really Does Matter when specifying CHP

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Courtesy of ENER-G

Discover the key points that CHP consultants need to consider in order to correctly size CHP units for optimum energy savings.

For CHP consultants and others involved in specifying cogeneration, it is essential that the CHP unit is correctly sized – in fact, it is crucial to a project’s viability. An under or oversized CHP unit will neither achieve the principal energy and cost savings, nor a satisfactory payback period.

An undersized CHP unit may operate at 100% output, but potential cost and CO2 savings are missed because of the shortfall in energy, meaning:

    • Additional electricity is imported from the National Grid.
    • Boilers are required for any shortfall in heat.
    • Payback periods are increased.

CHP consultants need to be aware that an oversized CHP unit may provide the full building load, but similarly will not deliver the potential savings:

    • The CHP will operate below its full output rating unless export of generated energy is an option.
    • Potential savings from ECAs, exemption from CCL and reduction in business rates will be missed if the scheme is not registered with the CHPA as Good Quality CHP.
    • Payback periods will be longer.

To size a CHP correctly for any site, CHP consultants should consider certain key points:

 1. Heat and electrical power profiles

The efficiency of existing systems needs to be established – checking if any further improvement measures can be identified. Reliable hourly data for electrical and heat demands is required to accurately determine the site’s energy profiles.

 2. Electrical and thermal load tracking

Fluctuations in electrical and thermal energy demands may be accommodated by setting the CHP unit to track or match the energy profiles by varying or modulating its output.

 3. Agreeing baseline energy costs

The CHP unit should be sized to operate at its optimum baseline electrical and thermal output. Agreement will be needed if it is necessary to:

    • Provide any shortfall in electricity from the grid and heat from on-site boilers.
    • Operate the CHP slightly above the thermal baseline to produce higher electrical output for increased financial savings.

4. To dump or not to dump?

Where the CHP operates above the thermal baseline, it may be necessary to ‘dump’ or reject any excess heat energy to keep the CHP operational. ‘Dumping’ is commonly achieved by using a suitably sized dry-air cooler/radiator.

 5. Future spark spread implications/sensitivities

Spark spread is the difference between the purchase price of the CHP fuel and the sell price of the electric power output and significantly affects the overall benefits of CHP. Carbon costs can have a profound effect on spark spread.

 6. Applying utility inflation

To ensure financial viability of CHP projects, CHP consultants need to carry out an analysis of the associated fuel and electricity tariff levels and calculate the effect of any potential inflationary rises.

7. Rules of thumb for savings calculations

    • CHP units need to operate with a high and constant heat demand for at least 4,500 hours per year.
    • CHP applications ideally require a spark spread of around 3 or more.
    • CHP needs to be classified as Good Quality CHP, meaning an electrical efficiency above 20% and a Quality Index rating exceeding 100. 

Don’t overlook the complex factors that need to be considered when specifying CHP. Download the  'Essential  Guide to Small Scale Combined Heat and Power'

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